Field of the Invention
The present disclosure relates to an image forming apparatus, for example, a copying machine, a printer, or a facsimile machine, and more particularly, to control of a fan motor provided inside an image forming apparatus.
Description of the Related Art
An image forming apparatus includes an exhaust fan motor (hereinafter referred to as “exhaust fan”) for suppressing an increase in temperature inside the image forming apparatus. The exhaust fan exhausts heated air inside the image forming apparatus. At a time of exhausting air, fine particles and the like may be discharged along with air. Such fine particles and the like are required to be reduced from the viewpoint of environmental protection. For example, in “Blue Angel”, German certification for products and services, a discharge amount of ultra-fine particles (UFPs), which are ultra-fine particles discharged from a product, is regulated. Therefore, the image forming apparatus is required to limit a UFP discharge amount by, for example, providing a dedicated filter in an exhaust path and exhausting air through the filter.
However, when air is exhausted through the filter, a loss of air flow occurs, and an effect of suppressing an increase in temperature inside the image forming apparatus is lowered. In order to lower the temperature inside the image forming apparatus in the same manner as in a case of providing no filter, for example, it is required to increase an air flow by increasing a rotation speed of the exhaust fan. However, as the rotation speed increases, the exhaust fan emits louder noise due to wind noise generated by itself and acoustic echo inside a duct. In order to solve those problems, it is required to control the rotation speed of the exhaust fan with high accuracy within a limited predetermined range of the rotation speed.
In order to control the rotation speed of the exhaust fan at a predetermined rotation speed, a pulse width modulation signal (hereinafter referred to as “PWM signal”) may be used. When the PWM signal is used, the exhaust fan has the rotation speed controlled by adjusting a cycle period of the PWM signal and a duty ratio of a pulse width. For example, in United States Patent Application Publication No. 2007/0098374 A1, there is disclosed a technology for performing feedback control for the exhaust fan by detecting a rotational speed of the exhaust fan and adjusting the duty ratio of the PWM signal in accordance with the detected rotational speed.
As the exhaust fan has a larger size, the air flow at the same rotation speed increases. The weight and inertia of the exhaust fan increase in proportion to a rotor size of the exhaust fan. Therefore, when the rotation speed is controlled to be constant by the PWM signal, the exhaust fan requires time until the rotation speed reaches a target rotation speed to become stable. When such feedback control as described above is started until the rotation speed becomes stable, there is a possibility that the duty ratio of the PWM signal may be adjusted before the rotation speed reaches the target rotation speed. Therefore, the feedback control is required to be started after the rotation speed reaches the target rotation speed and becomes stable.
Even when there is no change in duty ratio of the PWM signal, the exhaust fan may cause a variation in rotation speed due to individual differences in winding resistance value, rotor balance, and other such characteristics and tolerance of a power supply voltage. For example, the exhaust fan causes a variation of about ±10% in rotation speed due to the individual differences in characteristics, and a variation of about ±5% to 10% in rotation speed due to the tolerance of the power supply voltage. FIG. 7 is an explanatory graph of temporal changes in rotation speed exhibited until the rotation speed of the exhaust fan reaches the target rotation speed based on the PWM signal. In FIG. 7, a target control range is set to ±5% with respect to the target rotation speed. The variation in characteristics of the exhaust fan is ±15%. In this case, there is a possibility that the rotation speed may exceed the target control range during a speed stabilizing time period after the exhaust fan starts to rotate until becoming stable and during a period after the feedback control is started until the rotation speed is controlled to fall within the target control range.
The image forming apparatus includes a nonvolatile memory configured to store the duty ratio of the PWM signal for controlling the rotation speed to fall within the target control range. The duty ratio of the PWM signal is, for example, a duty ratio for correcting a difference between the rotation speed including a variation factor of the exhaust fan, which is measured before shipment, and the target rotation speed. The image forming apparatus controls the rotation of the exhaust fan by adjusting the PWM signal in accordance with the duty ratio stored in the nonvolatile memory.
However, when the exhaust fan or the power supply unit is replaced due to, for example, a failure, the variation factor of the rotation speed changes. In this case, there is a possibility that, with content stored in the nonvolatile memory, it may become unable to control the rotation speed of the exhaust fan to fall within the target control range. This necessitates processing for calculating the duty ratio of the PWM signal for correcting the difference from the target rotation speed in accordance with the rotation speed including the variation factor even when a component is replaced in the same manner in the case before shipment. However, work of this processing requires the speed stabilizing time period. This increases a time period required for replacement and adjustment performed after shipment.
There is also a possibility that the image forming apparatus may unintentionally discharge UFPs when an operation for image forming processing is started before the rotation speed of the exhaust fan reaches the target rotation speed. For example, when a rotational speed of the exhaust fan is lower than a speed of a target value, there is a possibility that an amount of air exhausted through the filter may decrease and a static pressure of an exhaust path may decrease, to thereby discharge UFPs from an unintended portion of the image forming apparatus. When the rotational speed of the exhaust fan is faster than the speed of the target value, the noise becomes louder.
For that reason, when the image formation is performed on condition that the rotation speed of the exhaust fan has been controlled to fall within the target control range by the feedback control, a large amount of time is required until the first printed product is output. Therefore, the present disclosure provides an image forming apparatus configured to appropriately control the rotation of an exhaust fan.